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BIG PHYSICS, BIG QUESTIONS –

Alien life could have basked in big bang’s afterglow

By Jacob Aron

Ancient alien life might have been incubating on strange exoplanets just 15 million years after the birth of the universe. Although nothing more complex than microbes could have existed back then, the possibility may poke holes in the notion that the universe is fine-tuned for human existence.

Astronomers hunting for signs of alien life today look for exoplanets within the habitable zone of stars – the region around a star that is warm enough for liquid water to exist on a planet’s surface. In the deep freeze of space, worlds that are too far from their host stars should be inhospitable to life as we know it.

But space wasn’t always so frigid, argues Abraham Loeb of Harvard University. The very early universe was filled with superheated gas, or plasma, that gradually cooled and condensed to form stars and galaxies. We see the first light emitted by this plasma as the cosmic microwave background (CMB).

Today, the CMB’s temperature is just a few degrees above absolute zero. But Loeb calculates that about 15 million years after the big bang, the radiation would have been warm enough to make the whole universe one large habitable zone. This life-friendly epoch would have lasted a few million years, enough time for microbes to emerge but not complex life, says Loeb.

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Stable home

A thornier issue is whether any planets could have formed so early in the universe’s history, along with the complex molecules necessary for life. When the hot plasma cooled, it initially produced only hydrogen and helium atoms. Heavier stuff had to be cooked up inside the nuclear forges of the first stars then expelled when those stars exploded.

Standard cosmology says that in most parts of the universe, the amounts of heavy elements needed to make planets didn’t occur until hundreds of millions of years after the big bang. But our current understanding of the early distribution of matter is incomplete, says Loeb. If some regions were much denser than average, it is possible stars and planets formed there earlier – perhaps in time for the CMB to be the right temperature. “These planets are very rare objects that are extremely unlikely, but because the universe is so large, you could still have them,” he says.

Such physical conditions might have been right for life, but any microbes would also need enough time to evolve, argues Jack O’Malley-James at the University of St Andrews, UK. Our mid-sized sun is already almost 5 billion years old and is expected to live for another 5 billion. By contrast, the earliest stars would have been much more massive with shorter lifespans, maybe only a few million years or so. “These systems would have to be very calm and stable from a very early stage to give life a good chance of gaining a foothold,” says O’Malley-James.

Great values

As wacky as the idea of such ancient life seems, Loeb thinks it is worth exploring if it puts a dent in the anthropic principle. This hotly debated idea in cosmology says that the fundamental constants in the universe are tuned in just the right way for us to be around to observe them.

For instance, a parameter called the cosmological constant accounts for the accelerated expansion of the universe due to dark energy. But theories predict that the value for this constant is more likely to be enormously larger than the one we have measured. If that were the case, the outwards push of dark energy should outmatch the pull of gravity, and structures like planets and galaxies should not have been able to form in the first place.

The anthropic principle solves this conundrum by assuming that the different possible values for the cosmological constant can appear across multiple universes. The value can also change with time in a given universe as it is tied to the density of matter, which changes as the universe expands. We happen to live in the universe that is suited to our existence at a time when the value of the constant is just right for human life to emerge and observe it. Many cosmologists don’t like this line of reasoning, though, as they believe it is too simplistic.

“The anthropic argument gives us an excuse for not seeking a more fundamental understanding,” says Loeb. That makes the notion of “big bang life” appealing. The denser regions of matter needed for it to arise would have also required a cosmological constant a million times larger than ours. That would mean life existed in our universe even at a time when the value of the cosmological constant would not have favoured humans, making our existence less special.

It is possible that life evolved during Loeb’s habitable epoch, agrees Alexander Vilenkin at Tufts University in Medford, Massachusetts. However, he reckons the odds of it happening are very low, and that most life in our universe should instead be suited to today’s conditions, so from a statistical view the anthropic principle lives on.

“If we are typical observers, we should expect to live in a region with a small cosmological constant,” Vilenkin adds.